But after starting just two league games since last summer’s high-profile move from Harlequins, the outsized Samoan prop is ready to make an impact.
Johnston concedes that the new scrum laws, allied to a different playing style, have made it difficult to settle into his new surroundings.
But he insists he is finally coming to terms with it, telling The Rugby Paper: “I struggled a bit in the first few months.
“Sarries wanted me to have more of a running game and I had to adjust to new systems and new plays, especially at the lineouts.
“Also, the new scrummaging laws had a big effect on the way I played and it took me a good while to get used to that as well.
“The laws are good, but there was a big adjustment with the bind and when you’ve been used to one thing, it takes time to adapt.
“Under the old laws there was more of a hit which, with my physique, suited my game more. But I feel I’m getting to grips with it now and I’ve worked hard with our coach, Alex Sanderson, and Matt Stevens to iron things out.”
Johnston, 27, revealed he has shed weight in order to improve mobility. “I’ve put on more muscle and lost a few kilograms by cutting down on stuff,” he said. “That’s enabled me to get around the field a lot better than I used to.
“I’ve been in and out of the team, which is different to Quins where I was starting every week.
“It’s meant coming off the bench a lot but hopefully I can get a few more starts.”
Johnston may have lost weight, but that has not stopped him from branching into business by launching his own pie-making company.
He explained: “It’s just a little venture I’m working on and Saracens helped me set it up.
“We make beef and cheese pies and I launched them (100 samples in Sarries’ Olympic Bar) at the London Irish game last week. It’s something I’m looking at for life after rugby.
“I’ve thought about it for a while so it’s nice to get the ball rolling. It hasn’t got a name yet but I’m thinking of ‘JJ’s Pies’.”
Saracens have plenty still to play for but Johnston admits last week’s home defeat by Irish gave his side a huge jolt.
He added: “It was a real wake-up call but we’re still well placed in three competitions.
“The main priority is the Premiership and we’ve got to focus fully on that before heading back into the LV= and Heineken Cups.
“From a personal perspective, I want to play well from now to the end of the season and be part of some success with Sarries. Then I hope to get back into the Samoa side for the Pacific Nations Cup, because the World Cup is a huge target for all us guys.”
NEALE HARVEY